Easy processing linear low density polyethylene

ABSTRACT

A polymer of ethylene and at least one alpha olefin having at least 5, carbon atoms obtainable by a continuous gas phase polymerization using supported catalyst of an activated molecularly discrete catalyst such as a metallocene in the substantial absence of an aluminum alkyl based scavenger which polymer has a Melt Index (MI) as herein defined of from 0.1 to 15; a Compositional Distribution Breadth Index (CDBI) as defined herein of at least 70%, a density of from 0.910 to 0.930 g/ml; a Haze value as herein defined of less than 20%; a Melt Index ratio (MIR) as herein defined of from 35 to 80; an averaged Modulus (M) as herein defined of from 20 000 to 60 000 psi (pounds per square inch) and a relation between M and the Dart Impact Strength in g/mil (DIS) complying with the formula: 
     
       
         DIS≧0.8×[100 +e   (11.71-0.000268×M+2.183×10     −9     ×M     2   )].

This is a divisional of Ser. No. 09/048,569, filed Mar. 26, 1998, nowU.S. Pat. No. 6,255,426, which claims the benefit of ProvisionalApplication No. 60/042,310, filed Apr. 1, 1997.

FIELD OF THE INVENTION

The invention relates to novel polyethylene compositions andparticularly to polyethylene having improved combination of shearthinning behavior (to assist in the processing of such polymers in themolten state) and impact strength (to assist the end-use performance).The polymers can be optimally produced in a continuous gas phaseprocesses in which supported catalyst is introduced into a fluidized bedreactor.

BACKGROUND OF THE INVENTION

Polyethylene produced from gas phase processes with a degree ofbranching to improve melt rheology are described in EP-A-495099;EP-A-452920; EP-A-676421 and EP-A-659773. WO 96/08520 (Exxon ChemicalPatents Inc) discusses gas phase polymerization using low concentrationsof scavenger, in other words, no or only a low amount of scavenger inthe form of, for example triethyl aluminum, is used in the course ofpolymerization.

Polyethylene with improved rheology obtained with mono-cyclopentadienylcompounds are described in WO-A- 93/08221.

U.S. Pat. Nos. 5,336,746; 5,525,689; and 5,639,842 (EP-A-495099) producepolyethylene using hafnium metallocene compounds having multidentateligands (i.e. they have two cyclopentadienyl ring systems connected by abridge). The specifically named hafnium compounds are bridged. Thedescribed polymerization is performed in a batch system. Thepolymerization is performed with unsupported catalyst in a solutionphase, although mention is made of vapor phase operation. The propertiesof the resulting polyethylene include a narrow molecular weightdistribution and a Melt Flow Rate (MFR expressed in g per 10 minutes at190° C. under a load of 2.16 kg) of from 8-50. The abbreviation MFR isused to indicate Melt Flow Rate or Melt Flow Ratio depending on thesource. Reference must be made to the original source in case of doubtto determine the meaning of MFR in a particular case.

U.S. Pat. No. 5,374,700 (EP-A-452920) does exemplify the use ofsupported catalyst for making polyethylene. The polymerization is in thegas phase using triisobutyl aluminum as a scavenger. The transitionmetal component includes zirconocenes. Example 9 and others useethylene-bridged bis(indenyl)zirconium as the transition metal compound.Example 10 uses an Al/Zr ratio of 112. The scavenger helps to avoid theeffect of adventitious poisons attached to the experimental equipment orintroduced with the various components. The melt tension is said to beimproved.

WO-A-95/07942 uses monocyclopentadienyl compounds in a gas phase on asupport for producing polyethylene. The activator is not methylalumoxane but a non-coordinating bulky anion first described in U.S.Pat. Nos. 5,278,119; 5,407,884; and 5,403,014 (EP-A-277003 andEP-A-277004). Polymerization was performed in a batch reactor. Scavengerwas not mentioned.

U.S. Pat. No. 5,466,649 describes in Example 17 preparing polyethyleneusing a batch gas phase polymerization procedure using dimethylsilylbis(tetrahydroindenyl) zirconium dichloride on one support and TMA(trimethyl aluminum) supported separately on another support. This was abatch reaction and no detailed indication of the polyethylene propertieswas given.

U.S. Pat. No. 5,763,543, incorporated by reference, (WO 96/08520)describes a continuous commercial gas phase operating process in whichscavenger is either not present or is present in a reduced amount. Oneembodiment (see page 12, line 28) defines a system essentially free ofscavenger, i.e. containing less than 10 ppm of scavenger based on thetotal weight of the feed gas, which is there referred to as the recyclestream. Alternatively, the low scavenger condition is defined inrelation to the metallocene. On page 14, a molar ratio is defined offrom 300 to 10. On page 15 it is indicated that the number of olefinicor unsaturated oligomers in the resulting polymer is greatly reduced.

EP-A-676421 exemplifies a batch type process and a continuous processfor producing polyethylene which leads to an improved rheology productthrough introduction of long chain branching by the use of a supportedbis-cyclopentadienyl transition metal compound having an alkylene orsilyl bridge used in conjunction with a methylalumoxane cocatalyst. Thebatch reactions are with a scavenger (see page 5, line 28). Example 10of this patent publication discloses an Melt Index (MI) of 0.3 g per 10minutes determined at 190° C. under a 2.16 kg load; there is noindication of the molecular weight distribution, the CompositionalDistribution is not given, the density is 0.916 g/ml, the Haze is 11%,there is no indication of the ratio of MI's determined under differentloads, the Dart Impact Strength is 210 g/mil and there is no indicationof the polymer stiffness as expressed by the modulus. On the basis ofthe correlation between density and secant modulus given in theEncyclopedia of Polymer Science and Engineering, by Mark, Bikales,Overberger, and Menges, Vol. 6, second ed., p.447 (1986), the secantmodulus for this material is estimated to be about 30,000 to 32,000 psi(205 to 220 N/mm 2).

EP-A-659773 discloses the use of bridged catalysts for the operation ofa reactor in a continuous mode for the polymerization of polyethylene.The specification discloses the use of support (see page 6 line 30) butthe examples do not use a support so that the alumoxane is in a solventwhen injected. The use of an unsupported catalyst may favor fouling andfurthermore the alumoxane will contain a significant amount of unreactedtrimethyl aluminum (TMA) which may act as a scavenger and lead to anapparent increase in vinyl unsaturation. Melt processing is furtherinfluenced by the use of more than one metallocene component which canbroaden the molecular weight distribution by the production of more thanone distinct polymer component. This is done allegedly to providecontrol over the degree of long chain branching (LCB) as indicated bythe degree of LCB determined by GPC and viscosity data. The melt flowratio (MFR) is also used to characterize the polymer. The MFR is theratio of melt index (MI) at different loads and reflects LCB and higherMw/Mn. Increasing MFR values may be due to higher Mw/Mn caused by theuse of more than one metallocene. The examples indicate that the bridgedspecies is most instrumental in raising the level of LCB. HoweverExample 5, which shows the use of the bridged metallocene alone,produces a polymer having a very low molecular weight, suggesting thatthe low molecular weight polymer species are a major contributor tohigher MFR values. EP-A-659773 thus fails to teach how a low melt indexmaterial may be produced which has the improved rheology as expressed inMFR resulting from the presence of LCB. EP-A-659773 does not disclosethe CDBI, haze and DIS values which help determine the commercialquality of the polymer produced.

EP-A-743327 describes the preparation of an ethylene polymer having ahigh polydispersity index (which can be represented by Mw/Mn) whichrequires a lower head pressure in extrusion. The improved rheologicalproperties are expressed in terms of RSI (Relaxation Spectrum Index)which is said to be sensitive to molecular weight distribution,molecular weight and long chain branching. The polymerization processdetails are scant. EP-A-743327 includes as catalyst similar metallocenesto those listed in EP-A-659773.

EP-A-729978 characterizes an ethylene polymer using flow activationenergy. The polymer is made using bridged bis cyclopentadienyl catalystcomponents, with one cyclopentadienyl ring system being a fluorenylpolynuclear ligand structure. The higher activation energy may be theresult of higher levels of long chain branching.

Many different process or catalyst options are introduced in the aboveprocesses to achieve the desired effect in the melt processing of theresulting polymers. However it is suggested that these processes allsuffer from drawbacks which mitigate against commercial implementationin that the catalyst may have low productivity, be prone to fouling inthe longer runs used for large scale reactors and/or produce lowmolecular weight materials. In addition the prior proposals may lead toan undue sacrifice of physical properties such as loss of clarity,increase in extractability, which is detrimental in food contactapplications, or loss of film toughness properties such as dart impactstrength (DIS).

It is amongst the aims of the invention to provide a relatively simpleprocess for providing commercially desirable polymer from commercialscale plants which has advantageous melt flow properties and balance ofstrength and stiffness.

The polymer can be produced in prolonged production runs underconditions not likely to lead to fouling.

SUMMARY OF THE INVENTION

The invention provides a polymer of an ethylene and at least one alphaolefin having at least 5 carbon atoms obtainable by a continuous gasphase polymerization using supported catalyst of an activatedmolecularly discrete catalyst in the substantial absence of an aluminumalkyl based scavenger (e.g., triethylaluminum (TEAL), trimethylaluminum(TMAL), tri-isobutyl aluminum (TIBAL), tri-n-hexylaluminum (TNHAL) andthe like), which polymer has a Melt Index (MI) as herein defined of from0.1 to 15; a Compositional Distribution Breadth Index (CDBI) as definedherein of at least 70%, a density of from 0.910 to 0.930 g/ml; a Hazevalue as herein defined of less than 20; a Melt Index ratio (MIR) asherein defined of from 35 to 80; an averaged Modulus (M) as hereindefined of from 20 000 to 60 000 psi (pounds per square inch) (13790 to41369 N/cm²) and a relation between M and the Dart Impact Strength ing/mil (DIS) complying with the formula:

DIS≧0.8×[100+e ^((11.71-0.000268×M+2.183×10) ⁻⁹ ^(×M) ² )],

where “e” represents 2.7183, the base Napierian logarithm, M is theaveraged Modulus in psi and DIS is the 26 inch (66 cm) dart impactstrength.

While many prior art documents describe processes and polymers using thesame monomers and similar processes, none describe polymers combining[A] good shear thinning and therefore relatively favorable extrusion andother melt processing properties with [B] a high stiffness and [C] highimpact strength. Up to now these features appeared to be difficult tocombine in LLDPE (linear low density polyethylene) materials produced ina continuous gas phase process. The invention provides a surprisingcombination of properties for the polymer which can be preparedreproducibly.

In comparison to LDPE (low density polyethylene) made in the highpressure process having a comparable density and MI, the polyethylenesof the invention have a favorable DIS-Modulus balance, e.g., a dartimpact strength (DIS) in g/mil that is greater than that predicted bythe formula:

DIS≧0.8×[100+e ^((11.71-0.000268×M+2.183×10) ⁻⁹ ^(×M) ² )],

where “e” is the base Napierian logarithm and M is the averaged modulusin psi and DIS is the minimum dart impact strength for the polymer ing/mil.

In comparison with LLDPE made by a gas phase process using conventionalZiegler Natta supported catalysts, the polyethylenes of the inventionhave improved shear thinning. These conventionally produced LLDPE's willhave a relatively low CDBI and a poor DIS-Modulus balance, e.g., a dartimpact strength in g/mil that is less than that predicted by the aboveformula.

In comparison to the EXCEED™ materials (made by Exxon Chemical) producedin gas phase processes using metallocene based supported catalysts, thepolyethylenes of the invention have a better shear thinning behavior andcomparable other properties. The MIR will be from 16 to 18 for suchEXCEED materials.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the relationship between the averagedmodulus in psi and the dart impact strength (DIS) in g/mil for a numberof polymers as well as depicting the formula:

DIS≧0.8×[100+e ^((11.71-0.000268×M+2.183×10) ⁻⁹ ^(×M) ² )],

where “e” is the base Napierian logarithm and M is the averaged modulusin psi and DIS is the dart impact strength of the polymer in g/mil.

DETAILED DESCRIPTION

In a preferred form of the invention, the polyethylenes of the inventionare derived from ethylene and up to 15 weight percent of 1-hexene.Preferably, the relation between the Modulus and the Dart ImpactStrength complies with the formula:

DIS≧2.0×[100+e ^((11.71-0.000268×M+2.183×10) ⁻⁹ ^(×M) ²)],

where “e” is the base Napierian logarithm and M is the averaged Modulusin psi and DIS is the dart impact strength of the polymer in g/mil.

Advantageously, the polymer may have either one or combination of thefollowing features: the density is from 0.915 to 0.927 g/ml, the MI isfrom 0.3 to 10 and CDBI is at least 75%. Most preferred is a DIS is from120 to 1000 g/mil, especially less than 800 and more than 150 g/mil.Preferably the Mw/Mn by GPC is from 2.5 to 5.5

As to the process conditions, the overall conditions described in U.S.Pat. No. 08/306,055 (WO 96/08520), incorporated by reference herein, canbe adopted. Inventors believe that a combination of particular processconditions helps to make the polyethylene of the invention. Inparticular, it is thought desirable to use a catalyst system in whichthe metallocene has a pair of bridged cyclopentadienyl groups,preferably with the bridge consisting of a single carbon, germanium orsilicon atom so as to provide an open site on the catalytically activecation. The activator may be methyl alumoxane as described in U.S. Pat.Nos. 5,324,800; 5,580,939; and 5,633,394, incorporated by referenceherein, (EP-129368) or a noncoordinated anion as described in U.S.patent application Ser. No. 08/133,480, incorporated by referenceherein, (EP-277004). It also thought desirable that there should besubstantially no scavengers which may interfere with the reactionbetween the vinyl end unsaturation of polymers formed and the openactive site on the cation. By the statement “substantially noscavengers” and “substantial devoid or free of Lewis acid scavengers”,it is meant that there should be less than 100 ppm by weight of suchscavengers present in the feed gas, or preferably, no intentionallyadded scavenger, e.g., an aluminum alkyl scavenger, other than thatwhich may be present on the support.

The conditions optimal for the production of the polyethylene of theinvention also require steady state polymerization conditions which arenot likely to be provided by batch reactions in which the amounts ofcatalyst poisons can vary and where the concentration of the comonomermay vary in the production of the batch.

Overall continuous gas phase process for the polymerization of apolyethylene may thus comprise:

continuously circulating a feed gas stream containing monomer and inertsto thereby fluidize and agitate a bed of polymer particles, addingmetallocene catalyst to the bed and removing polymer particles in which:

a) the catalyst comprises at least one bridged bis cyclopentadienyltransition metal and an alumoxane activator on a common or separateporous support;

b) the feed gas is substantially devoid of a Lewis acidic scavenger andwherein any Lewis acidic scavenger is preferably present in an amountless than 100 wt. ppm of the feed gas;

the temperature in the bed is no more than 20° C. less than the polymermelting temperature as determined by DSC, at a ethylene partial pressurein excess of 60 pounds per square inch absolute (414 kPaa), and

d) the removed polymer particles have an ash content of transition metalof less than 500 wt. ppm, the MI is less than 10, the MIR is at least 35with the polymer having substantially no detectable chain endunsaturation as determined by HNMR

By the statement that the polymer has substantially no detectable endchain unsaturation, it is meant that the polymer has vinyl unsaturationof less than 0.1 vinyl groups per 1000 carbon atoms in the polymer,e.g., less than 0.05 vinyl groups per 1000 carbon atoms, e.g., 0.01vinyl groups per 1000 carbon atoms or less.

The process aims to provide the polyethylene of the invention throughoutthe use of a single catalyst and the process does not depend on theinteraction of bridged and unbridged species. Preferably the catalyst issubstantially devoid of a metallocene having a pair of pi bonded ligands(e.g., cyclopentadienyl compounds) which are not connected through acovalent bridge, in other words, no such metallocene is intentionallyadded to the catalyst, or preferably, no such metallocene can beidentified in such catalyst, and the process uses substantially a singlemetallocene species comprising a pair of pi bonded ligands at least oneof which has a structure with at least two cyclic fused rings (e.g.,indenyl rings). Best results may be obtained by using a substantiallysingle metallocene species comprising a monoatom silicon bridgeconnecting two polynuclear ligands pi bonded to the transition metalatom.

The catalyst is preferably supported on silica with the catalysthomogeneously distributed in the silica pores. Preferably, fairly smallamounts of methyl alumoxane should be used, such as amounts giving an Alto transition metal ratio of from 400 to 30, and especially of from 200to 50.

In order to obtain a desired melt index ratio, the molar ratio ofethylene and comonomer can be varied, as can concentration of thecomonomer. Control of the temperature can help control the MI. Overallmonomer partial pressures may be used which correspond to conventionalpractice for gas phase polymerization of LLDPE.

The parameters used in the claims and the examples are defined asfollows

Melt Index: ASTM D-1238- Condition E

Melt Index ratio: this is ratio of 121 over 12 as determined by ASTMD-1238.

Mw, Mn and Mw/Mn: determined by GPC using a DRI (differential refractionindex) detector.

Gel permeation chromatography (GPC) is performed on a Waters 150C GPCinstrument with DRI detectors.

GPC Columns are calibrated by running a series of narrow polystyrenestandards. Molecular weights of polymers other than polystyrenes areconventionally calculated by using Mark Houwink coefficients for thepolymer in question.

CDBI is determined as set out in column 7 and 8 of W09303093 as well asin Wild et al, J. Poly. Sci., Poly. Phys. Ed., Vol. 20, p. 441 (1982)and U.S. Pat. No. 5,008,204, which are incorporated by reference herein.

SCB (short chain branching): This was determined by HNMR (hydrogennuclear magnetic resonance) with data collected at 500 Mhz. Spectra werereferenced by setting the polymer backbone signal to 1.347 ppm. Methylgroup contents in ethylene 1-olefin copolymers were calculated from theHNMR spectrum using the following formula:

Methyl Groups/1000 Carbons=(I_(CH3)*0.33*1000)/(I_(0.5-2.1ppm)*0.5)

where I_(CH3) is the normalized methyl signal area in the region between0.88 and 1.05 ppm and I_(0.5-2.1)ppm the area between 0.50 and 2.10 ppm.

The amount of methyl groups will correspond to the number of short chainbranches in the polymer assuming that the short chain branches contain 1methyl (—CH₃) group and that all methyl groups are a result of shortchain branching. The same NMR method can be used to determine vinyl endunsaturation.

Density: ASTM D-1505

Haze%: ASTM D-1003-95

Dart Impact Strength, 26 inch: ASTM D1709-91

1% secant Modulus: ASTM D-882-91

The “averaged Modulus” is the sum of the 1% secant Modulus in themachine direction and in the transverse direction divided by two.

Elmendorf tear strength ASTM D1922-94

Granular Bulk Density: The granular polymer particles are poured via a⅞′ diameter funnel into a fixed volume cylinder of 400 ml. The bulkdensity is measured as the weight of resin divided by 400 ml to give avalue in g/ml.

Particle Size: The particle size is measured by determining the weightof material collected on a series of U.S. Standard sieves anddetermining the weight average particle size in micrometers based on thesieve series used.

Extractability: determined according to FDA regulations 21CFR 177.1520(d) (3) (ii).

EXAMPLES

Two runs are illustrated in detail below.

Catalyst Preparation

Run 1 Supported Catalyst Preparation:

A solution of 1300 ml of 30 wt % alumoxane (MAO) in toluene asdetermined by reference to the total Al content, which may includeunhydrolyzed TMA was charged to a two gallon(7.57 Liter), jacketedglass-walled reactor, equipped with a helical ribbon blender and anauger-type shaft. 2080 ml of toluene was added and stirred. A suspensionof 31.5 g dimethylsilyl-bis-(tetrahydroindenyl) zirconium dichloride(Me₂Si(H₄Ind)₂ZrCl₂) in 320 ml of toluene purchased from Albemarle Labs,was cannulated to the reactor. An additional bottle of dry toluene (250ml) was used to rinse solid metallocene crystals into the reactor bycannula under nitrogen pressure. A color change from colorless toyellow/orange was noted upon addition of the metallocene to the MAOsolution. The mixture was allowed to stir at 69° F. (20.6° C.) for onehour, before being transferred to a four-liter Erlenmeyer flask undernitrogen. Silica (1040 g, Davison MS 948, 1.65 ml/g pore volume wascharged to the reactor. Half of the solution from the 4 liter Erlenmeyerflask was then transferred back to the 2 gallon (7.57 liter) stirredglass reactor. The reaction temperature rose from 70° F. (21.1° C.) to100° F. (37.8° C.) in a five minute exotherm. The balance of thesolution in the 4 liter Erlenmeyer was subsequently added back to theglass reactor, and stirred twenty minutes. Then, toluene was added (273ml, 238 g) to dilute the active catalyst slurry, and stirred anadditional twenty-five minutes. Antistat AS-990, a surface modifier madefrom ethoxylated stearylamine sold by Witco Chemical Corp. (7 g in 73 mltoluene) was cannulated to the reactor and the slurry mixed for thirtyminutes. Removal of solvent commenced by reducing pressure to less than18 inches of mercury (457 mmHg) while feeding a small stream of nitrogeninto the bottom of the reactor and raising the temperature from 74° F.(23.3 ° C.) to 142° F. (61.1 ° C.) over a period of one hour. Then fiveadditional hours of drying at 142° F. (61.1° C.) to 152° F. (66.7° C.)and vacuum which ranged from 5 inches to 22 inches Hg (127 to 559 mmHg)were used to dry the support and yield 1709.0 g of free-flowing activesupported catalyst material. Head space gas chromatograph (HSGC)measurements showed 13,000 weight parts per million (1.3 wt %) ofresidual toluene. A second drying step under stronger vacuum conditions,resulted in HSGC analysis measurement of residual toluene at 0.18%.Elemental analysis showed 0.40% Zr, 10.75% Al, 30.89% Si, 0.27% Cl,9.26% C, 2.05% H (all percentages shown herein are weight percent).

Run 2 Supported Catalyst Preparation:

A solution of 1125 ml of 30 wt % alumoxane (MAO) in toluene asdetermined by reference to the total Al content which may includeunhydrolyzed TMA was charged to a two gallon (7.57 liter), jacketedglass-walled reactor, equipped with a helical ribbon blender and anauger-type shaft. 1800 ml of toluene was added and stirred. A suspensionof 30.8 g dimethylsilyl-bis-(tetrahydroindenyl) zirconium dichloride(Me₂Si(H₄Ind)₂ZrCl₂) in 320 ml of toluene purchased from Albemarle Labs,was cannulated into the reactor. An additional 150 ml of toluene wasused to rinse solid metallocene crystals into the reactor by cannulaunder nitrogen pressure. A color change from colorless to yellow/orangewas noted upon addition of the metallocene to the MAO solution. Themixture was allowed to stir at 69° F. (20.6° C.) for one hour, beforebeing transferred to a four-liter Erlenmeyer flask under nitrogen.Silica (899 g, Davison MS 948, 1.65 ml /g Pore Volume,V.) was charged tothe reactor. Half of the solution from the 4 L Erlenmeyer flask was thentransferred back to the 2 gallon (7.57 liter) stirred glass reactor. Thereaction temperature rose from 70° F. (21.1 ° C.) to 100° F. (37.8° C.)in a five minute exotherm. The balance of the solution in the 4 literErlenmeyer was subsequently added back to the glass reactor, and stirredtwenty minutes. Then, toluene was added (273 ml, 238 g) to dilute theactive catalyst slurry, and stirred an additional twenty-five minutes.Antistat AS-990 was cannulated to the reactor and the slurry mixed forthirty minutes. Removal of solvent commenced by reducing pressure toless than 18 inches of mercury (457 mmHg) while feeding a small streamof nitrogen into the bottom of the reactor and raising the temperaturefrom 74° F. (23.3° C.) to 142° F. (61.1° C.) over a period of one hour.Then nine and a half additional hours of drying at 142° F. (61.1° C.) to152° F. (66.7° C.) at a vacuum which ranged from 5 inches to 22 inchesHg (177 to 559 mmHg) were used to dry the support and yield 1291.4 g offree-flowing active supported catalyst material.

Fluid-Bed Polymerization:

The polymerization was conducted in a continuous gas phase fluidized bedreactor having a 16.5 inch (41.9 cm) diameter with a bed height ofapproximately 12 feet (3.6 M). The fluidized bed is made up of polymergranules. The gaseous feed streams of ethylene and hydrogen togetherwith liquid comonomer were mixed together in a mixing tee arrangementand introduced below the reactor bed into the recycle gas line. Theindividual flow rates of ethylene, hydrogen and comonomer werecontrolled to maintain fixed composition targets. The ethyleneconcentration was controlled to maintain a constant ethylene partialpressure. The hydrogen was controlled to maintain a constant hydrogen toethylene mole ratio. The concentration of all the gases were measured byan on-line gas chromatograph to ensure relatively constant compositionin the recycle gas stream.

The solid catalyst was injected directly into the fluidized bed usingpurified nitrogen as a carrier. Its rate of injection was adjusted tomaintain a constant production rate of the polymer. The reacting bed ofgrowing polymer particles is maintained in a fluidized state by thecontinuous flow of the make up feed and recycle gas through the reactionzone. A superficial gas velocity of 1-3 ft/sec (0.3 to 0.9 m/sec) wasused to achieve this. The reactor was operated at a total pressure of300 psig (2068 kPa gauge). To maintain a constant reactor temperature,the temperature of the recycle gas is continuously adjusted up or downto accommodate any changes in the rate of heat generation due to thepolymerization.

The fluidized bed was maintained at a constant height by withdrawing aportion of the bed at a rate equal to the rate of formation ofparticulate product. The product is removed semi-continuously via aseries of valves into a fixed volume chamber, which is simultaneouslyvented back to the reactor. This allows for highly efficient removal ofthe product, while at the same time recycling a large portion of theunreacted gases back to the reactor. This product is purged to removeentrained hydrocarbons and treated with a small stream of humidifiednitrogen to deactivate any trace quantities of residual catalyst andcocatalyst.

TABLE 1 Polymerization Run Condition Run 1 Run 2 Zr (wt %) 0.43 0.50 Al(wt %) 11.6 11.4 Al/Zr (mole/mole) 91.2 77.1 Temperature (° C.) 79.4 85Pressure (bar) 21.7 21.7 Ethylene (mole %) 25.0 49.9 Hydrogen (mole ppm)275 445 Hexene (mole %) 0.23 0.32 Bed Weight (Kg PE) 113 121 ProductionRate (Kg 27.6 35.5 PE/Hr) Catalyst Productivity 1690 2287 (Kg PE/Kgcatalyst) Bulk Density (g/ml) 0.448 0.450 Average Particle Size 920 803(micronmeters) Ash (ppm) 507 386

The parameters were determined as set out previously; the Zr, Al wtpercent and ash levels were by elemental analysis.

No aluminum alkyl compounds were added to the reactor as scavenger. Theruns were continued for around 3 days.

The polymers resulting were subjected to additional tests, whereappropriate after first forming the polymer into film.

Polymer Characterization

TABLE 2A Run 1 Run 2 Density 0.9190 0.9257 MI 1.10 0.62 MIR 46.0 57.6DRI Detector Mw 92,200 104700 Mn 18,300 17900 Mz 208,400 287500 Mw/Mn5.04 5.85 DSC 2nd Melt-See note 1 1st melting peak. (C) 108.6 122.6 2nd.melting peak. (C) 119.3 117.3 CDBI 86 83.10 SCB (/1000 C) 15.4 10.6 wt %C6 9.3 6.4 mole % C6 3.3 2.2 Note 1: the sample had been molten and beenallowed to cool once previously

Film Characterization

TABLE 2B Run 1 Run 2 Blow up ratio 2.5 2.5 Gauge in mil (1 2.1 2.0 mil =25.4 micronmeter) 1% Secant Modulus, psi (N/cm²) 29420 45070 MD (machinedirection) (20284) (31075) TD (transverse direction), psi 31230 47420(N/cm²) (21532) (32695) MD + TD average, psi (N/cm²) 30325 46245 (20908)(31885) Elmendorf tear strength (g/mil) MD 207 134 TD 430 477 26 inch(66 cm) Dart Impact 410 156 Strength in g/mil Calculated DIS as afunction of 294 123 Modulus as per the formula DIS ≧ 0.8 × [100 +e^((11.71−) ^(0.000268×M+2.183×10) ⁻⁹ ^(×M) ² ⁾], where ^(M) is averagedModulus in psi and DIS is the (26 inch) dart impact strength in g/milHAZE (%) 10.2 9.9 Extractability 1.0 not available

A larger number of further tests were performed with different samplesmade according to the invention in a similar manner and the results areshown in the Drawing 1. The function in the claim 1 is shown as a solidline.

Table 3 shows some exemplary values.

TABLE 3 Dart Impact Strength (26 inch) Calculated from Formula Averaged1% Secant DIS ≧ 0.8 × [100 + e^((11.71−) Modulus, M as Measured^(0.000268×M+2.183×10) ⁻⁹ ^(×M) ² ⁾] Measured psi N/cm² g/mil g/mil25,575 17,633 508 611 28,580 19,705 353 456 28,990 19,987 337 553 29,14520,094 332 451 30,325 20,908 294 410 31,450 21,684 264 284 31,610 21,794260 257 32,000 22,063 251 349 32,140 22,159 248 223 33,780 23,290 217251 34,160 23,552 211 262 35,170 24,248 196 223 35,970 24,800 186 26137,870 26,110 167 251 39,325 27,113 155 197 39,390 27,158 154 193 43,67530,112 131 167 46,245 31,884 123 156 47,730 32,908 119 147 49,460 34,101115 143

The claims therefore cover the combination of DIS and averaged Modulusin the area of the Drawing above the solid line.

Using the indications and guidance provided in the specificationconcerning catalyst selection, catalyst support and gas phase processoperation it is possible to produce ethylene polymers as specified inthe claims which are simultaneously optically clear; relatively easy tomake and to process and have a high strength as measured by the DartImpact Strength.

The films can be used for heavy duty bags, shrink film, agriculturalfilm, particularly which are down-gauged such as garbage and shoppingbags with a thickness of from 0.5 to 7 mil. The films can be produced byblow extrusion, cast extrusion, co-extrusion and be incorporated also inlaminated structures.

We claim:
 1. A continuous gas phase process for the polymerization ofethylene and an alpha-olefin, the process comprising continuouslycirculating a feed gas stream comprising ethylene and at least onealpha-olefin having from 5 to 20 carbon atoms to thereby fluidize andagitate a bed of particles, adding metallocene catalyst to the fluidizedbed, and removing polymer produced thereby, wherein the polymer producedhas a melt index of from 0.1 to 15, a compositional distribution breadthindex of at least 70%, a density of from 0.910 to 0.930 g/ml, a hazevalue of less than 20%, a melt index ratio of from 35 to 80, an averagedModulus (M) of from 20,000 to 60,000 psi, and a relation between M andthe dart impact strength in g/mil (DIS) complying with the formula:DIS≧0.8×[100+e ^((11.71-0.000268×M+2.183×10) ⁻⁹ ^(×M) ² )].
 2. Theprocess of claim 1, wherein the catalyst comprises at least one bridgedbis-cyclopentadienyl transition metal and an alumoxane activator on acommon or separate porous support.
 3. The process of claim 1, whereinthe feed gas stream is substantially devoid of a Lewis acidic scavenger.4. The process of claim 1, wherein the fluidized bed has a temperatureof no more than 20° C. less than the polymer melting temperature asdetermined by differential scanning calorimetry, at an ethylene partialpressure in excess of 60 psi absolute.
 5. The process of claim 1,wherein the polymer product has an ash content of transition metal ofless than 500 wt. ppm, the melt index is less than 10, and the meltindex ratio is at least
 40. 6. The process of claim 1, wherein thepolymer product has substantially no detectable chain end unsaturation.7. The process of claim 1, wherein the catalyst is substantially devoidof a metallocene having a pair of pi-bonded ligands which are notconnected through a covalent bridge.
 8. The process of claim 7, whereinthe catalyst comprises a substantially single metallocene speciescomprising a pair of pi-bonded ligands, at least one of which has astructure with at least two cyclic fused rings.
 9. The process of claim8, wherein, the substantially single metallocene species comprises amono-atom silicon bridge connecting two polynuclear ligands pi-bonded tothe transition metal atom.